Apparatus for making ultrafine calcium sulfate dihydrate crystals



. March 7,1967 MCCLEARY ETAL 3,307,919

APPARATUS FOR MAKING ULTRAFINE CALCIUM SULFATE DIHYDRATE CRYSTALSOriginal Filed Aug. 1, 1960 I mum/T0125.

Wm mo SMZQW the slurry.

United States Patent M 3,307,919 APPARATUS FOR MAKING ULTRAFINE CAL-CIUM SULFATE DIHYDRATE CRYSTALS Robert E. McCleary, Geneva, and WilliamA. Kinkade,

Lisle, Ill., assignors to United States Gypsum Company, a corporation ofDelaware Original application Aug. 1, 1960, Ser. No. 50,816, new PatentNo. 3,262,799, dated July 26, 1966. Divided and this application Jan.10, 1966, Ser. No. 519,565 7 Claims. (Cl. 2328'7) This application is adivision of the copending application of Robert E. McCleary and WilliamA. Kinkade, Serial Number 50,816, filed August 1, 1960, and now PatentNo. 3,262,799, issued July 26, 1966, and entitled, Method and Apparatusfor Making Ultrafine Calcium Sulfate Dihydrate Crystals.

This invention relates to an apparatus for making ultrafine calciumsulfate dihydrate crystals. This invention more particularly relates toan apparatus for the production of ultrafine crystals of calcium sulfatedihydrate in aqueous suspension, especially adaptable as an acceleratorto hasten the setting process of plaster of Paris, commonly known asstucco by those skilled in the art.

The time within which a slurry of calcined gypsum can be made to set hasalways played a large part in mechanization of the process ofmanufacture of fabricated gypsum products. Many attempts have been madeto provide a low cost, eificient accelerator that could be added to aslurry of calcined gypsum to cause the slurry to set within a period ofa few minutes. The normal set time within which a slurry will solidifywithout an accelerator is known to be in the order of 40 minutes; thusattempts have been made to reduce the normal setting time to speed upmanufacturing processes. It is well known that various chemical compoundadditives will substantially reduce the set time of calcined gypsumslurry, and some of the more common compounds now used to reduce thesetting time are potassium sulfate, ammonium sulfate, and finely groundcalcium sulfate dihydrate among others. These compounds, thoughefficient by themselves in reducing the set time, are many times toouneconomical to be practical.

Generally, in the gypsum industry, the most economical and practicalmethod of speeding the set time of a slurry of calcined gypsum, prior tothis invention, was to add finely dry-ground crystalline calcium sulfatedihydrate to The calcium sulfate dihydrate acts to initiate therehydration of the hemihydrate in the slurry, and the most economicalmethod of obtaining the calcium sulfate dihydrate prior to thisinvention was by grinding a block or cast of set gypsum. Frequently asupplementary portion of a chemical additive accelerator, such asammonium sulfate or potassium sulfate, etc., before mentioned, was alsoadded to achieve proper setting action.

The ground block process left much to be desired for many reasons.Perhaps the primary disadvantage of using ground block process was itspredisposition to cause a lack of uniformity in the ultimate set gypsumproduct. Another disadvantage in using ground block as an acceleratorwas the tendency of the block to have a certain amount of free watercontent, since these blocks are often incompletely dried. The freewater, of course, made it impossible to obtain efiiciency of grindingdue to the tendency of the block material to collect on the grindingwheels and plug the abrasive surface. Perhaps an even more seriousdifficulty encountered in the grinding of these blocks was calcinationdue to the heat of grinding. Samples of the ground calcium sulfatedihydrate often showed up to 30% calcination with consequent loss orvariation in potency, and the product was also subject .to rapid loss ofefficiency when exposed to the atmosphere.

3 ,307,919 Patented Mar. 7, 1967 Also, this gypsum block grindingprocess necessitated manual servicing and monitoring by a responsibleindividual, and this process, therefore, did not lend itself Well toautomation. This crude manner of providing an accelerator thus left agreat deal to be desired.

In view of the disadvantages noted with the use of ground gypsum blockas an accelerator, it has long been desired to eliminate the processentirely and this is one of the primary objectives of this invention.

Prior to this invention, attempts were made to provide ultrafine calciumsulfate dihydrate crystals by other processes. Various batch processeswere devised including at least one wherein calcium sulfate dihydratecrystals were produced from an aqueous suspension. Such attempts,however, were unsuccessful inasmuch as extremely fine crystals, i.e.,below 1 micron in diameter, could not be made thereby, and the crystalsproduced were not small enough to form a practical setting acceleratorfor calcined gypsum slurries.

It is very desirable to have the accelerator material in an extremelyfine form so that it may be uniformly and efficiently distributedthroughout the slurry which is to be set. The finer the material, thegreater the efficiency, and, therefore, the less material required toset a given quantity of slurry. For mechanization it is, of course, alsodesirable that the accelerator production process be continuous.

Therefore, one of the objects of this invention is to provide anapparatus that will promote the rate of nucleation and control thesubsequent growth of ultrafine calcium sulfate dihydrate crystals in anaqueous medium to facilitate the production of an extremely potent setaccelerator for a slurry in which calcined gypsum is the majorcomponent.

Another object is to provide an apparatus for promoting and controllingthe rate of nucleation of ultrafine calcium sulfate dihydrate crystals,which apparatus is especially adaptable to mechanization of the processof making set gypsum products.

Another object is to provide an apparatus for producing an acceleratorwhich may be more uniformly distributed throughout a slurry of unsetcalcium gypsum.

Further objects and advantages will become obvious to those skilled inthe art upon reading the following detailed description and claims inconnection with the accompanying drawings.

In the drawings:

FIGURE 1 is an elevational view of one embodiment of the crystalnucleating apparatus of this invention; and

FIG. 2 is an enlarged sectional elevation view of a portion of thecrystal nucleating apparatus, taken substantially along line 2-2 of FIG.1.

Referring now more particularly to the drawings, FIG. 1 is anillustration showing the various mechanisms which may be employed. Theapparatus is illustrated in connection with the manufacture of gypsumboard.

A portion of stucco (plaster of Paris, consisting essentially of thehe-mihydrate of calcium sulfate) is diverted to a stucco hopper 3.Hopper 3 is provided with a feeder mechanism 4 that causes a constantflow from the bot tom of hopper 3. The feeder mechanism 4 may be a screwfeeder such as that manufactured by Vibra Screw Feeders, Inc. ofClifton, New Jersey, and it may be equipped with a remote control speedadjustment to enable the operator to control this and other mechanismsused in the wallboard process from a centralized control panel. Adjacentthe hopper 3 is situated chemical additive hopper 5, which is designedto contain a quantity of additive such as ammonium sulfate, potassiumsulfate, finely ground calcium sulfate dihydrate or other acceleratorcompounds. The additive hopper 5 is also provided with a feedermechanism 6 similar to that used on the mechanism 4 on the stuccohopper.

The hoppers 3 and 5 are situated so as to permit the feed from thefeeder mechanisms 4 and 6 respectively into a mixing receptacle 8. Thereceptacle 8 is preferably conical in shape, and a float level tank It)may be connected thereto either directly as shown, or by means of a pipeor hose. The float level tank 10 is kept filled by a float valve 12connected to a pressurized water source 14. A tangential fluidconnection 16 is provided between the mixing receptacle 8 and the floatlevel tank 10 near the base of the receptacle 8. This tangentialconnection provides a circulating vortex of water to keep the interiorwalls of mixing receptacle 8 clean. This connection from float leveltank 10 to mixing receptacle 8 may be varied to provide the mostefficient water flow, or if desired, other means, such as a mechanicalagitator or rotary scraper blade, may be substituted to preventaccumulations. A metering pump 18 connected to the bottom of the mixingreceptacle 8 provides positive and uniform flow of the dilute suspensionconsisting of hemihydrate, water and, if used, an auxiliary acceleratoradditive into a nucleating or microcrystallizing apparatus 20'.

Apparatus 20 is comprised of an elongate, substantially closed,cylindrical or tubular container 22 having a hollow agitator shaft 24disposed longitudinally therein for relative rotation with respectthereto. A suitable power means 26 is connected to and effects rotationof the shaft 24 within the container 22. The tubular container 22 isdisposed horizontally and is fixed in position, so that the shaft 24disposed therein rotates relatively thereto. Mounted longitudinally onshaft 24 are a plurality of impeller bars or ribs 28 which arecircumferentially spaced about the periphery of the shaft 24 andprotrude widthwise radially outward from the shaft. As best illustratedin FIG. 2, the preferred embodiment includes six such impeller bars 28,and each bar has its forward or leading face sloping or pitchedrearwardly with respect to the direction of shaft rotation. The shaft 24with the impeller bars 28 attached thereto is adapted to rotate in thedirection indicated by the arrow in FIG. 2. It has been found that bestresults are obtained when the impeller bars 28 protrude radially fromthe periphery of the shaft 24 a distance which is at least 10% of thediameter of the shaft 24.

The container 22 is substantially filled with steel balls, which arepreferably in the order of /2 inch in diameter, and the clearancebetween the outer extremities of the impeller bars 28 on the rotatingshaft 24 and the wall of the surrounding container 22 should be inexcess of two ball diameters to prevent binding. Thus, if /2 inchdiameter balls are used, this clearance should be at least 1 inch. Thenumber of steel balls within the container 22 is necessarily limited bythe size of the container, the space within the container occupied bythe shaft 24 and the impeller bars 28 and by the requirement that theshaft and impeller bars must be permitted to rotate within the containerafter the container has been loaded with balls. It is important to notethat the major function of the microcrystallizer is not to grind butrather to divide and agitate, so that the dihydrate crystal nucleationis promoted while the crystal growth is substantially retarded. Theresult is unusually fine crystals most of which are less than 1 micronin diameter.

The nucleating or microcrystallizing apparatus 20 discharges through itsoutlet 32 into the Water line 34, which conveys water from a gaugingwater tank 36 to the mixer manifold 38 of a gypsum wallboard makingmachine. The rate of discharge of the dihydrate suspension from themicrocrystallizer is governed by the rate at which the hemihydratesuspension is fed into the microcrystallizer by the metering pump 18.The water from the .gauging tank 36 and the suspension from themicrocrystallizer 20 are pumped through the line 34 and through ll 6tromagnetic flow meter 40 in that line by means of a positivedisplacement pump 42. The gauging water line 34 is connected to thegypsum board mixer manifold 38 by means of a three-way valve 44, and aline 46 extends from this three-way valve 44 so that when the valve isin one of its positions, the water and the dilute suspension may passthrough the line 34, through the valve 44 and be disposed of through theline 46 which may lead to the drain or sewer. The pump 42 pulls waterfrom the gauging water tank 36 so that the total amount of water fromthe tank 36 and the microcrystallizer 20 is the amount of water desiredto be introduced into the mixer manifold 38. The electromagnetic flowmeter 40 serves as a monitor of the water flow rate and indicates theamount of liquid passing through the line 34.

The apparatus is used for making ultrafine crystals of calcium sulfatedihydrate as follows: A portion of stucco is placed in the hopper 3, anda quantity of ancillary accelerator material, such as ammonium sulfateor finely ground calcium sulfate dihydrate, is placed in additive feeder5. A prescribed quantity of ancillary accelerator material and stuccoare fed into mixing receptacle 8 and mixed thoroughly with water priorto being fed into the microcrystallizer 20 by the pump 18. In themicrocrystallizer 20 the dilute suspension is minutely divided, and theagitation and impaction of the rotating ball mass within themicrocrystallizer induces rapid solution of the calcium sulfatehemihydrate and extensive nucleation of calcium sulfate dihydratecrystals. The action of the balls, the diluteness of the solution, andthe fairly rapid rate of flow through the microcrystallizer 20, tend todiscourage the joining of the nuclei and tend to minimize the size ofthe crystals formed. The net result is a large number of tiny crystalsof calcium sulfate dihydrate, the majority being below 1 micron indiameter. The suspension of tiny dihydrate crystals thus formed iscontinuously discharged into water line 34. This suspension iscontinuously forced into the mixer manifold 38 by the .positivedisplacement pump 42. When the operation is initiated, the three-wayvalve 44 is opened to the waste line 46 and closed to the mixer manifold38 for a period in the order of five minutes, which is usuallysufficient to completely clear the microcrystallizer 20. This allows thesystem to achieve equilibrium with the desired concentration of crystalswithin. The invention will be more fully understood by the followingexample.

The operation of the apparatus is continuous, and it is preferred thatthe suspension be passed through the microcrystallizer at such a ratethat it remains in the microcrystallizer for from three to ten minutes,with about five minutes being preferred. This rate will, of course,depend upon many things including the makeup of the stucco, the amountand type of auxiliary accelerator used, if any, the water used, etc.

Example 1.This is an example of the invention practiced in conjunctionwith a machine making /2-inch thick, 4-feet wide gypsum board andoperating at 104 lineal feet per minute. Stucco normally used in gypsumboard manufacture was continuously fed from the hopper 3 by feedermechanism 4 which was adjusted to de liver 3.1 pounds per minute ofstucco to the wet mixing receptacle 8. Ancillary chemical additive,i.e., ammonium sulfate in this case, was provided in hopper 5 and wascontinuously fed to mixing receptacle 8 by feeder mechanism 6 at therate of 0.25 pounds per minute. Water at F. was continuously andautomatically supplied from float tank 10 through the connection 16 tothe mixing receptacle 8 at the rate of 108 pounds per minute. The water,additive, and stucco making up a dilute suspension was drawn from mixingreceptacle 8 and metered into one end of the microcrystallizer 20 by thepositive displacement pump 18 at the rate of about 111.35 pounds perminute.

The dilute suspension was passed through the microcrystallizer 20 in ahelical path and. discharged at the opposite end thereof at a uniformrate of flow. While in the microcrystallizer 20, the suspension wasintensively agitated by the rotating mass of balls 30 which contributedto the rapid dissolution of the stucco and additive and promotednucleation and formation of ultrafine crystals of calcium sulfatedihydrate. The resulting aqueous suspension of ultrafine crystals wasdischarged from the microcrystallizer into the gauging water line 34.

In the line 34 the dilute aqueous suspension of ultrafine dihydratecrystals joined with the gauging water from the tank 36 and was passedthrough the positive displacement pump 42, the electromagnetic flowmeter 16 and through the three-way diverter valve 44 to the mixermanifold 38. The mixer manifold 38 was located on and supplied water tothe main gypsum board slurry mixer (not shown). Subsequent blending ofthe water from mixer manifold 38 with the main stream of dry stucco andadditive in the gypsum board slurry mixer distributed the acceleratingdihydrate crystals uniformly throughout the resulting gypsum boardslurry mass.

In the foregoing example, the amount of stucco used to form crystals ofdihydrate for acceleration was 7.46 pounds per thousand square feet andthe amount of ammonium sulfate additive was 0.625 pound per thousandsquare feet. These amounts produce a satisfactory setting action, and itwas not necessary to vary the stucco from the normal 3.1 pounds perminute. In the above example, feeder mechanism 5 was adjusted so as tofeed 2.5 pounds per minute of ancillary accelerator additive from hopper4 for the first five minutes of operation and was automatically adjustedto revert to 0.25 pound per minute thereafter. The first five minutes ofoperation in which the water and suspension were diverted to the drainwas called the dosing period and was utilized in order to promote rapidinitial nucleation in the microcrystallizer 20 and to minimize the timerequired to achieve an optimum concentration of accelerator crystals; orput it another way, the time was used to bring the microcrystallizer toequilibrium conditions prior to starting the board making process.

The microcrystallizer 20, in the above example, was about 14 feet long,and the container 22 had a 14-inch inside diameter. The shaft 24,extending through the microcrystallizer container 20 was about 8 inchesin diameter, and the impeller bars 28 extending from the outer peripheryof shaft 24 were backwardly sloped and extended radially in the order of1 inch from the outer periphery of the shaft 24. The container 22 wasloaded with 2,750 pounds of /z-inch steel balls each having a diameterof about /2 inch, and the power means 26 used to turn shaft 24 was a 40hp. electric motor. The shaft was rotated at a speed of about 110revolutions per minute.

It is not necessary that there be an additive accelerator compoundutilized; however, the microcrystallizer 20 can be made of considerablysmaller size, i.e., in the order of 40% smaller, if an additive is used.Thus, the preferred method would include the use of an acceleratoradditive. Also, although the aqueous suspension of ultrafine crystalswas added to the gauging water in the example, there is no reason whythe aqueous suspension could not be added directly to the slurry mixer38. The example shows the suspension added to the gauging water.

It may be noted that the optimum efficiency of making ultrafine crystalsof calcium sulfate dihydrate can be obtained only by taking intoconsideration a number of variables. The specific apparatus used is, ofcourse, varied in size to meet specific conditions, as above mentioned.An important variable is the available water supply, which may contain agreat deal of organic and/ or inorganic impurities. Certain impuritiesin the water may vary the efficiency of production of the desiredultrafine calcium sulfate dihydrate crystals within themicrocrystallizer 20. Another variable is the stucco itself. It

is very difficult to find two sources of gypsum stucco that areidentical in susceptibility to acceleration. Thus, the nature of thestucco must be taken into consideration, and it is extremely difiicultto generalize when referring to stucco.

When using the ultrafine crystals produced by the apparatus of thisinvention as an accelerator for a slurry in which calcined gypsum is thepredominant solid material, the stucco used to make the ultrafinecrystals may be as little as of the weight of the solids in the slurryto which the accelerator is added; however, it is generally between A;and A and is preferably about /2%. The accelerator should, however, notexceed 3% of the solids in the slurry used to make the gypsum product.

The amount of water that is generally added to the stucco used to makeultrafine crystals may also vary within an extremely large range. Thewater is always in excess of ten times the weight of the stucco and mayvary from a ratio of above 10 to l, i.e., more than 10 parts water to 1part stucco, to about 330 to 1 ratio or more. In a preferred embodimentof our method, the quantity of water is in excess of a 20 to 1 ratiowith the stucco and rarely to exceed to 1 ratio with the stucco. When anaccelerator compound is used, it is generally in the order of of theweight of the stucco used to make the ultrafine crystals. This amountwill, of course, vary somewhat in each individual case.

The variables above mentioned are given in an attempt to show howoptimum efliciency may be gained by the use of the apparatus of theinvention. It may be necessary in given instances to make allowance forspecific problems that reside in a certain given location.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that many modifications .and changes invarious details may be resorted to without departing from the spirit andscope of the invention, as hereinafter claimed.

What is claimed is:

1. An apparatus for producing ultrafine calcium sulfate dihydratecrystals comprising, an elongate substantially closed container, havingan inlet adjacent one end and :an outlet adjacent the opposite end, ashaft mounted longitudinally within said container for relative rotativemovement with respect thereto, said shaft carrying a plurality ofelongated longitudinally extending impeller ribs the transverse portionsof which protrude radially outward from said shaft, a plurality offreely movable elements disposed within and substantially filling saidcontain-er circ-umjacent said shaft, means for continuously feeding anaqueous charge of calcium sulfate hemihydrate and water in through theinlet of said container, and means for effecting rotation of said shaftwithin said container, whereby the aqueous charge is subjected todivision, agitation and impact and the product is continuouslydischarged from said outlet.

2. The apparatus of claim 1, wherein said elements are balls.

3. The apparatus of claim 2, wherein the spacing between the interiorwall of said container and the radial extension of said elongated,longitudinally extending imgeller ribs is equal to at least twice thediameter of said 4. The apparatus of claim 1 wherein said container ishorizontally disposed and stationary.

5. An apparatus for producing ultrafine calcium sulfate dihydratecrystals comprising, a substantially closed and horizontally-disposedelongate stationary container, having an inlet adjacent one end and anoutlet adjacent the opposite end, an elongate member mountedlongitudinally within said container for relative rotative movement withrespect thereto, said member having a plurality of elongatedlongitudinally disposed impeller portions peripherally spaced andprotruding radially a greater portions of said member intermediate saidimpeller portions, a plurality of balls freely movable within andsubstantially filling said container circumjacent said member, means forcontinuously feeding an aqueous charge of calcium sulfate hemihydrateand water in through the inlet of said container at a rate sufiicient tosubstantially fill void spaces between said balls, and means foreffecting rotation of said member within said container, whereby theaqueous charge is subjected to division, agitation and impact as itprogressively advances through said container, and the product iscontinually discharged from said outlet.

6. Apparatus of claim 5, wherein each impeller por- 8 tion has itsleading face vsloping rearwardly with respect to the direction ofrotation of said member.

7. The apparatus of claim 5, wherein the radial protrusion of each ofsaid impeller portions is at least ten percent of the smallesttransverse dimension of said member.

References Cited by the Examiner UNITED STATES PATENTS 2,779,752 1/1957Vining.

MORRIS O. WOLK, Primary Examiner.

JAMES H. TAYMAN, ]R., Examiner.

1. AN APPARATUS FOR PRODUCING ULTRAFINE CALCIUM SULFATE DIHYDRATECRYSTALS COMPRISING, AN ELONGATE SUBSTANTIALLY CLOSED CONTAINER, HAVINGAN INLET ADJACENT ONE END AND AN OUTLET ADJACENT THE OPPOSITE END, ASHAFT MOUNTED LONGITUDINALLY WITHIN SAID CONTAINER FOR RELATIVE ROTATIVEMOVEMENT WITH RESPECT THERETO, SAID SHAFT CARRYING A PLURALITY OFELONGATED LONGITUDINALLY EXTENDING IMPELLER RIBS THE TRANSVERSE PORTIONSOF WHICH PROTRUDE RADIALLY OUTWARD FROM SAID SHAFT, A PLURALITY OFFREELY MOVABLE ELEMENTS DISPOSED WITHIN AND SUBSTANTIALLY FILLING SAIDCONTAINER CIRCUMJACENT SAID SHAFT, MEANS FOR CONTINUOUSLY FEEDING ANAQUEOUS CHARGE OF CALCIUM SULFATE HEMIHYDRATE AND WATER IN THROUGH THEINLET OF SAID CONTAINER, AND MEANS FOR EFFECTING ROTATION OF SAID SHAFTWITHIN SAID CONTAINER, WHEREBY THE AQUEOUS CHARGE IS SUBJECTED TODIVISION, AGITATION AND IMPACT AND THE PRODUCT IS CONTINUOUSLYDISCHARGED FROM SAID OUTLET.